1. Field of the Invention
The present invention relates to a feedhorn, a radio wave receiving converter and an antenna, and particularly, to a feedhorn including a dielectric, a radio wave receiving converter and an antenna.
2. Description of the Background Art
Conventionally, an antenna for receiving a radio wave of satellite broadcasting or the like is known. To the antenna, a radio wave receiving converter is arranged. As a member constituting the radio wave receiving converter, a feedhorn in which a dielectric is connected to an open end of a waveguide is known (for example, see Japanese Patent Laying-Open No. 2001-217644).
According to Japanese Patent Laying-Open No. 2001-217644, a portion of a dielectric is press-fitted with the internal circumference of the open end of a waveguide, whereby the dielectric is fixedly connected to the waveguide.
With the feedhorn of such a structure, however, high dimensional precision of the internal circumference of the waveguide and the portion of the dielectric must be retained, otherwise the reliability of a connecting portion between the waveguide and the dielectric cannot be maintained. Additionally, the strength of the connecting portion between the waveguide and the dielectric may possibly change if the dielectric expands or shrinks due to heat, affected by changes in the ambient temperature. This will also degrade the reliability of the connecting portion. As a result, the reliability of a converter including the feedhorn, and hence, of an antenna including the converter may be degraded.
Additionally, since an antenna is often installed outdoors, a cover is provided as a protective member covering the dielectric, in order to protect the dielectric of the feedhorn from the external environment. Conventionally, the material forming the cover has been different from that forming the dielectric. Therefore, in order to prevent the cover from providing a negative effect to electric characteristics (such as radiation characteristics) of the dielectric, a space has been provided between the dielectric and the cover. As a result, the cover that is considerably larger than the dielectric has been required. Such a large cover has been hindering the converter including the feedhorn, and hence the antenna including the converter, in reducing their size.